MICR 3060 Final Exam Questions and
Answers
Describe the characteristics of UV light as a microbial control agent. Know how it affects DNA. - Answer-
Ultraviolet light is a form of electromagnetic radiation between 100-400nm in wavelength. UV light
causes the formation of thymine dimers in bacterial DNA where 2 thymine base pairs are covalently
bonded, causing breakage of DNA. However, it's only effective in sterilizing surfaces.
Describe how we examined the germicidal effects of UV light on S. aureus and B. subtilis. - Answer-A
nutrient agar plate was divided in half with one side streaked with B. subtilis and the other streaked with
S. aureus. half of each section was covered with a paper towel and put under UV light for variable time.
Generally, the half covered with the paper towel saw much more growth as UV was blocked, while
longer incubation times under UV light yielded far less growth.
Define the process of ammonification and explain how we tested for the presence of this process in the
soil. - Answer-Ammonification is the process by which organic nitrogen of decaying plants and animals
is converted back into ammonia. We tested for ammonification in soil by inoculating a peptone broth
tube with a sample and observing after incubation for 7 days at room temp.
Nessler's reagent was then added to the inoculation and color changes were observed. A reddish-brown
color should result in the presence of ammonia.
, Define and describe biofilms. - Answer-Biofilms are a complex assemblage of microbes encased in an
adhesive matrix and attached to a surface. They usually are present within aqueous environments and
provide a place of protection for the microbes. Biofilms also contribute to increased virulence.
Distinguish between antimicrobial agents, disinfectants, antiseptics, and antibiotics - Answer-
Antimicrobial agent - Substance that kills or inhibits growth of microorganisms
Disinfectant - Substance used to kill microorganisms on non-living surfaces
Antiseptic - A substance used to kill microorganisms on skin/ human surface tissue.
Antibiotic - Chemicals that kill or inhibit bacteria and treat bacterial infections. Can be synthetic or
natural.
Explain the Filter Paper Disk Method of evaluating the relative effectiveness of various disinfectants
and/or antiseptics. - Answer-Disks are impregnated with different antimicrobial substances and placed
onto a nutrient agar plate already inoculated with a bacterium/microbe. The zone of inhibition (area of
no growth around the disk) is measured after incubation for 24-48 hours. The larger the zone of
inhibition, the more effective the antimicrobial is.
Explain the Kirby-Bauer Method of evaluating the sensitivity of bacteria to certain antimicrobial drugs. -
Answer-Using the zone of inhibitions measured from the Filter Paper Disk Method and disk
concentration, the resistance level of the antimicrobial can be evaluated as resistant, intermediate, or
sensitive.
Define zone of inhibition and explain how its measurement can be used to determine if a bacterium is
Resistant, Intermediate, or Susceptible (Sensitive) to an antimicrobial agent. - Answer-The zone of
inhibition is the area of zero growth around a plated disk that is soaked in an antimicrobial agent. Using
the correlated table, the concentration of the microbial agent, matched with measured zone of
inhibition can be used to determine if the bacteria are resistant, intermediate or sensitive.
Answers
Describe the characteristics of UV light as a microbial control agent. Know how it affects DNA. - Answer-
Ultraviolet light is a form of electromagnetic radiation between 100-400nm in wavelength. UV light
causes the formation of thymine dimers in bacterial DNA where 2 thymine base pairs are covalently
bonded, causing breakage of DNA. However, it's only effective in sterilizing surfaces.
Describe how we examined the germicidal effects of UV light on S. aureus and B. subtilis. - Answer-A
nutrient agar plate was divided in half with one side streaked with B. subtilis and the other streaked with
S. aureus. half of each section was covered with a paper towel and put under UV light for variable time.
Generally, the half covered with the paper towel saw much more growth as UV was blocked, while
longer incubation times under UV light yielded far less growth.
Define the process of ammonification and explain how we tested for the presence of this process in the
soil. - Answer-Ammonification is the process by which organic nitrogen of decaying plants and animals
is converted back into ammonia. We tested for ammonification in soil by inoculating a peptone broth
tube with a sample and observing after incubation for 7 days at room temp.
Nessler's reagent was then added to the inoculation and color changes were observed. A reddish-brown
color should result in the presence of ammonia.
, Define and describe biofilms. - Answer-Biofilms are a complex assemblage of microbes encased in an
adhesive matrix and attached to a surface. They usually are present within aqueous environments and
provide a place of protection for the microbes. Biofilms also contribute to increased virulence.
Distinguish between antimicrobial agents, disinfectants, antiseptics, and antibiotics - Answer-
Antimicrobial agent - Substance that kills or inhibits growth of microorganisms
Disinfectant - Substance used to kill microorganisms on non-living surfaces
Antiseptic - A substance used to kill microorganisms on skin/ human surface tissue.
Antibiotic - Chemicals that kill or inhibit bacteria and treat bacterial infections. Can be synthetic or
natural.
Explain the Filter Paper Disk Method of evaluating the relative effectiveness of various disinfectants
and/or antiseptics. - Answer-Disks are impregnated with different antimicrobial substances and placed
onto a nutrient agar plate already inoculated with a bacterium/microbe. The zone of inhibition (area of
no growth around the disk) is measured after incubation for 24-48 hours. The larger the zone of
inhibition, the more effective the antimicrobial is.
Explain the Kirby-Bauer Method of evaluating the sensitivity of bacteria to certain antimicrobial drugs. -
Answer-Using the zone of inhibitions measured from the Filter Paper Disk Method and disk
concentration, the resistance level of the antimicrobial can be evaluated as resistant, intermediate, or
sensitive.
Define zone of inhibition and explain how its measurement can be used to determine if a bacterium is
Resistant, Intermediate, or Susceptible (Sensitive) to an antimicrobial agent. - Answer-The zone of
inhibition is the area of zero growth around a plated disk that is soaked in an antimicrobial agent. Using
the correlated table, the concentration of the microbial agent, matched with measured zone of
inhibition can be used to determine if the bacteria are resistant, intermediate or sensitive.
